1. Field of the Invention
The present invention relates to a crystallization apparatus in which semiconductor film is irradiated a non-crystallized with a laser beam to form a crystallized semiconductor film, an optical member for use in the crystallization apparatus, and a crystallization method. The present invention particularly relates to a crystallization apparatus in which a non-crystallized semiconductor film is irradiated with a laser beam phase-modulated using an optical modulating member such as a phase shift mask or plate to form a crystallized semiconductor film, and a crystallization method.
2. Description of the Related Art
For example, a semiconductor material of a thin film transistor (TFT) for use in a switching element which controls voltage applied to a pixel of a liquid crystal display (LCD) has heretofore been roughly classified into an amorphous semiconductor such as a-silicon and poly-semiconductor such as poly-silicon.
Poly-silicon has a higher electron mobility than that of amorphous silicon. Therefore, when poly-silicon is used to form the transistor, a switching speed is faster than that with the use of amorphous silicon. Therefore, response of the display is accelerated, and there is an advantage that a design margin of another component can be reduced. With the use of poly-silicon, when not only a display main body but also peripheral circuits such as a driver circuit and DAC are incorporated in the display, these peripheral circuits can be operated at a higher rate.
Poly-silicon is constituted of a grope of crystal grains, but is lower in electron mobility than single crystal silicon. In a small-sized transistor formed using poly-silicon, a dispersion of the number of crystal grain boundaries in a channel portion is a problem. To solve the problem, in recent years, there has been proposed a crystallization method of generating poly-silicon having a large grain size in order to enhance the electron mobility and to reduce the dispersion of the number of crystal grain boundaries in the channel portion.
As this type of crystallization method, “phase control excimer laser annealing (ELA)” has heretofore been known in which a polycrystalline semiconductor film or an amorphous semiconductor film is irradiated with an excimer laser beam via a phase shift mask to generate a crystallized semiconductor film. Details of the phase control ELA are disclosed in “Surface Science Vol. 21, No. 5, pp. 278 to 287, 2000”.
In the phase control ELA, an intensity distribution of an inverse peak type (intensity distribution in which light intensity rapidly rises as distant from a position where the light intensity is minimum) is generated by the phase shift mask. The polycrystalline semiconductor film or the amorphous semiconductor film is irradiated with a light beam which periodically has this inverse peak type intensity distribution. As a result, a molten region is generated in accordance with the light intensity distribution, and a crystal nucleus is formed in a non-molten portion or a first coagulated portion disposed opposite to a position where the light intensity is minimized. When a crystal grows in a lateral direction toward periphery from the crystal nucleus (lateral growth), the crystal having a large grain size is generated.
As described above, in the related art, the semiconductor film is irradiated with the light beam which has the light intensity distribution of the inverse peak type, the crystal nucleus is formed in the portion disposed opposite to the position where the light intensity is minimized in the intensity distribution, and therefore the control of the formed position of the crystal nucleus is possible. However, it is impossible to control the intensity distribution in an intermediate portion between two inverse peak portions display adjacent to each other.
In actual, in the related art, the intensity distribution in the intermediate portion generally involves an irregular surge (undulated distribution in which increase and decrease of the light intensity are repeated). In this case, in a process of crystallization, the lateral growth started from the crystal nucleus toward the periphery stops in a portion in which the light intensity decreases in the intermediate portion, and there is a disadvantage that a large crystal is inhibited from growing. Even if a substantially homogeneous intensity distribution is obtained in the intermediate portion, the lateral growth stops in an arbitrary position of this homogeneous intensity distribution, and there is a disadvantage that the growth of the large crystal is inhibited.